When you are getting sick, your heart rate variability (HRV) usually drops and your resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate goes up, sometimes a few days before you feel ill. HRV cannot tell you what is wrong or how serious it is, so your symptoms, not the number, should decide what you do next.
Why illness lowers HRV
Fighting an infection is hard work for the body. Fever pushes your heart rate up, and the parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system system, which normally slows the heart and lets the beat-to-beat rhythm vary, has less say. With a faster, steadier heartbeat, HRV falls.
There is also a direct link between the nerve that slows your heart and the immune system. The vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve , the main nerve of the parasympathetic system, also carries signals that help keep inflammation in check, a link researchers call the inflammatory reflex (Pavlov and Tracey, 2012).
What wearable studies found
Some of the largest studies of HRV during everyday illness were done during the COVID-19 pandemic, when many people already wore a device.
A large Fitbit study. Researchers funded by Fitbit analysed data from 2,745 people with COVID-19 confirmed by PCR test (Natarajan and colleagues, 2020). They measured RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD each night between midnight and 7 a.m. Around the start of symptoms, breathing rate and heart rate went up and RMSSD went down, and these changes could begin a few days before symptoms. Breathing rate showed the largest change and took the longest to return to normal. A model built on these signals predicted whether someone was ill on a given day with moderate accuracy, well short of a reliable test. In the weeks after, heart rate on average dipped below normal and returned to its usual level by about day 21, while HRV sat slightly above normal for part of that time.
Health care workers with Apple Watches. A study in New York followed hospital staff who wore an Apple Watch and answered daily symptom surveys (Hirten and colleagues, 2021). The daily pattern of SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , another HRV measure, changed in the week before and the week after a COVID-19 diagnosis, and on the first day people reported symptoms.
Both studies report average changes across groups, so they cannot tell you how your own numbers will behave. A hard workout or a few drinks can also produce a similar pattern.
Go deeper HRV after COVID-19
A German study measured 24-hour HRV with a Holter monitor (a portable ECG) in 103 people with post-COVID-19 syndrome, about 252 days after their infection (Mooren and colleagues, 2023). Compared with 90 healthy people, after adjusting for age and sex, they showed a shift toward the sympathetic side and lower parasympathetic activity at night. People who had been in hospital during the infection showed the largest shift.
This is a group comparison of people who already had lasting symptoms. It does not mean that low HRV after an infection points to long COVID, and HRV cannot be used to diagnose it. The authors call for longer studies to see whether HRV tracks recovery.
How to read your data when you’re sick
- Look at several signals together. A drop in HRV alongside a higher resting heart rate or breathing rate fits the pattern seen in the studies better than a drop in HRV alone. See HRV vs resting heart rate.
- Compare with your baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline . A reading well below your usual range for two or three nights with no other cause, such as alcohol or a hard session, is worth noting. The lesson on building your HRV baseline shows how to find that range.
- Tag sick days. Log symptoms, fever and medicines. It helps you see how long your own numbers take to settle, and keeps sick days from skewing your baseline later.
- Expect a slow return. Numbers can take days to weeks to come back after an infection. A return to baseline does not by itself mean you are fully recovered.
- Don’t use HRV to rule illness in or out. For more on how far HRV can warn you early, see can HRV predict illness?
When to talk to a doctor
Go by how you feel and what your symptoms are, whatever your wearable says. Get medical help if you have trouble breathing, chest pain, confusion, a high fever that does not come down, or symptoms that get worse instead of better. Talk to a doctor if fatigue, a racing or irregular heartbeat, dizziness when you stand up, or shortness of breath last for weeks after an infection.
If you have a heart condition or another long-term illness, ask your doctor how to use wearable data when you are unwell. If your HRV stays low for a long time without a clear reason, that is also worth mentioning to a doctor, along with any symptoms. What low HRV can mean covers the common causes.